Time division switching system employing
common transmission highways

ABSTRACT

IN A TIME DIVISION SWITCHING SYSTEM, SPEECH SAMPLES FROM A SUBSCRIBER STATION ARE FIRST ENCODED AND THEN STORED, IN DIGITAL CODED FORM IN A REGISTER IN A TRANSCEIVER. THE OUTPUT OF THIS REGISTER IS GTED TO A SECOND REGISTER IN THE COMMON TALKING BUS OR HIGHWAY DURING A FIRST PORTION OF A TIME SLOT, THE SAMPLE FROM THIS REGISTER IN THE COMMON HIGHWAY IS GATED IN A LATER PORTION OF THE SAME TIME SLOT TO A SIMILAR TRANSCEIVER CONNECTED TO THE CALLED SUBSCRIBER. IT IS THEN SUBSEQUENTLY DECODED AND APPLIED   TO THE CALLED SUBSCRIBER. TWO-WIRE,FOUR-WIRE, AND CONFERENCING ARRANGEMENTS ARE DISCLOSED.

Aug. 6, 1.14 w, NORDQUIST ETAL Re. 28,09

TIME DIVISION SWITCHING SYSTEM EIPLOYING common 'rmmsmsszou HIGHWAYSOriginal Filed July 25, 1969 8 Sheets-Sheet l.

TRANS- CEIVER TRANS- CElVER 30,. TRANS TRANS L |3| LATOR LATOR MEMORYREGISTER I40 i SWITCH| STORE W R. NORDQU/ST INVENTg/iS; m N

Q LQVJM A TTORNEY Aug. 6, 1974 w. R. NORD TIME DIVISION QUIST ET LSWITCHING SYSTEM EIIPLOYING COMIION TRANSMISSION HIGHWAYS Original FiledJuly 25, 1969 8 Sheet s-Sheet 2 ham MIC.

w 6t 8 8 3 220882; w 83 E2;

"5225 84 o2 8 N 6Q mczoummoz z .Al m 2 C Aug. 6, 1,74 w. R. NORDQUISTErAL Re. 28,097

TIME DIVISION SWITCHING SYSTEM EIPLOYING COMMON TRANSMISSION HIGHWAYSOriginal Filed July 25, 1969 8 Sheets-Sheet 5 g- 1974 w. R. NORDQUISTRe. 28,09!

TIME DIVISION SWITCHING SYSTEM EIIPLOYING COMMON TRANSMISSION HIGHWAYSOriginal Filed July 25, 1969 8 Sheets-Sheet &

8 Sheets-Sheet. 5

HHMUH HHWHH ILWmlIIILEIf l i T IIII l so fi $581 22:: I o: 4 I EEQZEL@2355 T 5 P 3 J 2: mon

Aug. 6, 1974 w. R. NORDQUIST I'AL TIME DIVISION SWITCHING SYSTEMEIPLOYING COMMON TRANSMISSION HIGHWAYS 5, 1969 Original Filed July 2 oonAug. 6, 1974 w. R. NORDQUIST EI'AL Re. 28,097

TIME DIVISION swrrcnzue SYSTEM summme common 'mmsmssmu HIGHWAYS OriginalFiled July 25, 1969 8 Sheets-Sheet 6 FIG. 7

Aug. 6, 1974 w. R. NORDQUIST EIAL Re. 28,091

TIME DIVISION SWITCHING SYSTEM EHPLOYING COMMON TRANSMISSION HIGHWAYSOriginal Filed July 25, 1969 8 Sheets-Sheet 7 Aug. 6, 1,14 w, NQRDQUISTETAL Rt. 28,091

TIME DIVISION SWITCHING SYSTEM EIPLOYING COMMON TRANSMISSION HIGHWAYSOriginal Filed July 25, 1969 8 Sheets-Sheet s United States PatentOffice Re. 28,097 Reissued Aug. 6, 1974 28,097 TIME DIVISION SWITCHINGSYSTEM EMPLOY- ING COMMON TRANSMISSION HIGHWAYS Walter ReinholdNordquist, Naperville, and Wing Noon Toy, Glen Ellyn, Ill., assignors toBell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Original No. 3,617,643, dated Nov. 2, 1971, Ser. No. 844,945, July 25,1969. Application for reissue June 11, 1973, Ser. No. 369,094

Int. Cl. H04] 3/00 US. Cl. 179-15 AQ 6 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE In a time division switching system, speechsamples from a subscriber station are first encoded and then stored, indigital coded form in a register in a transceiver. The output of thisregister is gated to a second register in the common talking bus orhighway during a first portion of a time slot; the sample from thisregister in the common highway is gated in a later portion of the sametime slot to a similar transceiver connected to the called subscriber.It is then subsequently decoded and applied to the called subscriber.Two-wire, four-wire, and conferencing arrangements are disclosed.

BACKGROUND OF THE INVENTION Telephone systems currently are availablewhich operate on a time division basis in which a number ofconversations share a single communication highway. Privacy ofconversation is assured in such systems by the division or separation ofindividual conversations in time. Thus each conversation is assigned tothe common highway for an extremely short, periodically recurringinterval, and the connection between any two lines in communication iscompleted only in the assigned interval or time slot. Samples whichretain essential characteristics of the voice or other signal aretransmitted over the common highway in these time slots and are utilizedin the called line to reconstruct the original signal.

One such telephone system, termed the electronic private branch exchangesystem, and designated PBX hereinafter, comprises a plurality ofindependent switch units, each located on a customers premises andinterconnecting the customers distinct group of telephone lines in pairsvia a time division switching network and a common transmission highway.All of the switch units are served in common by a remote control unitwhich, because of its electronic components, can tolerate many times theamount of traffic that a single switch unit can provide. switch unit insuch a system is dictated instead by the nature of the internal timedevisions switching operation.

The sampling frequency in a time division multiplex system must be atleast twice the highest frequency of the signal to be transmitted. Forvoice, this requires a sampling rate of approximately 8 kilohertz, andtypical PBX systems employ 24 time slots recurring at l25-microsecondintervals termed frames, the time slot duration being 5.2 microseconds.In switching these signal samples between telephone lines incommunication, the energy transferred during operation of the timedivision switch must be approximately equal to the energy in the signalbetween successive samples in order to permit faithful reproduction ofthe original signal at the receiving terminal. As the time slot durationis made smaller, more power must be transmitted through the timedivision switch, and it is this situation which provides the ultimatelimitation on the maximum number of time slots per frame. Such alimitation, of course, severely restricts the size of a switch unit on acustomers premises, and expansion beyond that limit may requireadditional switch units which in turn adds to the expense of the PBXfacility.

SUMMARY OF THE INVENTION In accordance with one illustrative embodimentof our invention, the problem encountered with respect to the capacityof a PBX switch unit on a customers premises is solved by employment ofa unique switching arrangement which permits a reduction in the durationof the time division switching operation itself, thus realizing asignificant increase in the number of conversations which may time-sharethe common transmission highway. Signals from a subscriber station aresampled at a rate which is at least twice the highest frequencycomponent. Thus, assuming the highest frequency in a speech signals is 4kilohertz, sampling will be effected at an S-kilohertz rate.

The speech samples then are coded and applied to a transceiver which maycomprise a first register having sufiicient capacity to accommodate allof the digits in the coded sample and being capable of bilateraltransmission. The output of the transceiver is applied to anotherregister on a common transmission highway via a bidirectionaltransmission gate which is enabled selectively during a brief portion ofa time slot. The sample on the common highway is transferred throughanother bidirectional gate, in a later portion of the same time slot, toanother transceiver connected to the called subscriber station.Subsequent to this time slot, the sample is decoded and applied to thecalled subscriber station. A similar path exists for transmission ofsignal samples from the called station to the calling station includinga second common highway. Such an arrangement permits a reduction in theduration of a time slot to the extent that, with a sampling rate of 8kilohertz, more than 1,000 time slots may be served within a repetitivel25-microsecond frame. This contrasts with the permissible 24 time slotsin prior art systems at the same sampling rate.

In accordance with another illustrative embodiment of our invention, asimilar operation is performed in a two wire system as contrasted withthe four-wire arrangement considered hereinbefore. The distinction restsin the fact that separate paths exist for incoming and outgoing signalsin the four-wire system while the two-wire system contemplates that thesame path will be shared by incoming and outgoing signals. Occasionallyin telephone conversations both parties will be speaking simultaneously.In this event the signal transmitted over the common high way in thetwowire system will be a composite of the signals produced at bothcalling and called stations. The composite signal available on thetwo-wire path to each of the calling and called subscribers then isaltered by a subtraction operation prior to decoding, such that only thedesired signal is applied to each of the calling and called stations.The two-wire operation differs from the four-wire operation in theprovision of this subtraction technique in each subscriber line plus theaddition of the coded signal samples present on the two common highways.The resultant composite signal is returned to both calling and calledlines via the corresponding highways by directional gates andtransceivers.

The method of addition and subtraction depends upon the technique usedin quantizing the input signal for encoding in binary form. For example,if the quantization is logarithmic, the addition and subtraction must bedone logarithmically.

In accordance with one respect of the two-wire arrangement, conferenceconnections may be completed in a similar fashion. Thus the compositesignal received from all conferees may be accumulated during severaltime slots and then returned simultaneously to the conferees. Thus theresultant composite signal is simply the summation of all signalsreceived in the common highways during the previous sequence of timeslots. A distinct advantage derived from this arrangement is evidentfrom the fact that any number of subscribers may be involved is aconference connection without loss or degradation of signals.

THE DRAWINGS FIG. I is a block diagram of a communication system inaccordance with one illustrative embodiment of the invention;

FIG. 2 is a time chart illustrating the operations occurring during asingle time slot in the system depicted in FIG. 1;

FIG. 3 is a block diagram of a communication system in accordance withanother illustrative embodiment of the invention;

FIG. 4 is a time chart illustrating the operations occurring during asingle time slot in the system depicted in FIG. 3; and

FIGS. 5-9 are block diagrams of systems of the type depicted in FIG. 3modified to accommodate conference connections and various otheroperations.

DETAILED DESCRIPTION Turning now to the drawing, the time divisiontelephone system depicted in FIG. 1 is similar to the system disclosedin R. C. Gebhardt et al. US. Pat. No. 3,225,144, issued Dec. 21, 1965,which will be described in general terms hereinafter to provide a basisfor the detailed description of the improvements realized in accordancewith this invention.

A PBX switch unit is illustrated which, in accordance with the Gebhardtet al. disclosure, provides the necessary switching and controlfacilities to accommodate a number of telephone stations 100-1 through100-n. A remote control unit, not illustrated, processes all informationpro vided by the illustrated switch unit in order to establish alldesired call connections through the switch unit. The informationnecessary to the establishment of the actual time division switchingconnections is transferred from the remote control unit to the switchstore 150 at the switch unit where it is included in a list of gatecontrol messages which are cyclically scanned and applied individuallyin a regular sequence to memory register 140. Thereafter, during thetime slot assigned to the particular conversation designated by themessage in register 140, the message is translated into gate controlsignals by translators 130 and 131 to provide a sequence of gate controlsignals which enable transmission gates represented by gates 107, 107,108 and 108'. Individual samples of the voice signal provided by thecalling and called stations active in this particular conversation arethus transferred between the stations via common transmission highways115 and 120.

For ease of illustration, only two of the stations 100- 1 through 100-nare illustrated together with the necessary circuits to permitcommunication between them. The arrangement in the aforementionedGebhardt et al. patent contemplates a maximum of 24 simultaneousconversations, each individual conversation being sampled simultaneouslyat the calling and called stations and the resultant signal sample beingtransferred between the respective participants via a commontransmission highway during a preassigned one of 24 time slots in arecurrent cycle or frame. An 8-kilohertz time division sampling rate maybe employed in this system in order to obtain a transmission qualityequivalent to a 3.5 kilohertz nonmultiplexed arrangement. At thissampling rate, each of 24 speech sample periods or time slots has aduration of 5.2 microseconds. Such a duration is required to assureproper transfer of the speech samples through the time divisionswitching network and faithful reproduction of the original signal atthe receiving terminal.

In accordance with this embodiment of our invention, the number ofavailable time slots may be increased by an order of magnitude, and thisoperation will be described hereinafter. The facilities required in thetransmission path include encoders 101 and 103, which convert theoriginal voice sinal to a pulse code modulated (PCM) form. Typically,the number of bits required to enode voice signal into PCM form variesbetween four and eight depending upon the quality desired. In thefollowing description it is assumed that in bits are utilized. In orderto transfer in bits simultaneously from transceivers and 106 toregisters 116 and 121, control gates 107, 107, 108 and 108' and highwaysand also each accommodate m bits. Similarly, on the receiving end,decoders 102 and 104 restore the coded voice signals to analog formprior to receipt by the corresponding stations 101-1 and 100-n. Suchencoders and decoders for conversion between analog and PCM may take anyone of a number of forms available in the art. Transceivers 10S and 106comprise simple shift registers having a number of register stagescorresponding to the number of digits in each coded speech sample. Againsuch as register is well known in the art.

Transmission gates 107, 107, 108 and 108' are of a form for rapidtransfer of digital information between transceivers 105, 106 and thecommon transmission highways 115, 120. Advantageously, these gatesshould be capable of transmitting digital information in eitherdirection as will be evident in considering the various arrangementsdepicted in FIGS. 5-9. Such a bidirectional gate is known in the art asdisclosed, for example. in W. R. Nordquist et al. patent application,Ser. No. 787,- 185, filed Dec. 26, l968, now Pat. No. 3,618.033.Transmission highways 115 and 120 are m bit lines which each receive asignal sample in a first portion of a time slot and apply the signalsamples to the respective registers 116 and 121, which again are ofconventional form.

The prior art switching arrangement as described in the aforementionedGebhardt et al. patent utilizes a single common transmission highwaywhich receives a signal sample from a calling station during one timeslot and transmits that sample to the called station during the sametime slot. In this instance, therefore, a pair of m bit transmissiongates, each corresponding to one of the calling and called stations,operates simultaneously to effect the desired signal transfer betweenthe two stations. As noted in FIG. 1, the arrangement in accordance withthis embodiment of our invention stores each signal sample transferredto one of the two highways 115 and 120 during a first portion of a timeslot and directs these stored samples to the appropriate receivingstations in a later portion of the same time slot.

This distinction may be understood by considering the operationsinvolved in transferring samples between two stations in communication,viz, stations 100-1 and 100-n. It is assumed that the remote controlunit has honored the request for service received from one of these twostations and has provided the necessary information to switch store 150for the establishment of a connection between these two stations duringa preassigned time slot. Thus as illustrated in FIG. 1, switch store 150contains the designations of the appropriate gates which must operateduring the preassigned time slot to effect a signal transfer from thesetwo stations.

Consider, for example, that a signal sample emanates from stations 100-1and 100-n simultaneously. These samples are converted from analog to PCMform in the respective encoders 101 and 103 and subsequently are appliedto transceivers 105 and 106. At the next appearance of the assigned timeslot, the information designated in switch store 150 will be applied tomemory register 140, converted to a gate control pulse in translatorsand 131 and applied to gates 107 via lead 132 and to gates 108 via lead135 so as to operate these gates simultaneously. With gates 107 and 108enabled, the coded signal samples are transferred from transceivers 105and 106 to common highways 115 and 120 respectively for registration inthe corresponding registers 116 and 121. After a brief delay and stillwithin the preassigned time slot interval, translators 130 and 131provide control signals simultaneously on leads 133 and 134, serving toenable the respective gates 108 and 107'. This operation results in thetransfer of signal samples from registers 116 and 121 to transceivers106 and 105, respectively. Thereafter the signal samples are convertedto analog form in decoders 102 and 104 and applied to the respectivestations 1001 and 100-n.

It should be noted that the paths traveled through the switching networkby the respective signal samples are unilateral. However, thetransceivers are each employed only for a brief portion of the entireframe by the corresponding station. Therefore, each transceiver may betime shared by a number of stations on a time multiplex basis. This, ofcourse, reduces drastically the number of transceivers and transmissiongates required in this arrangement. With unilateral transmission it isevident that two paths through the network are involved in each activeconversation, one path for transmitting or talking and another path forreceiving or listening. Prior to the sampling period, the encodedsamples provided by station 100-1 and l00-n are stored in theirrespective transceivers 105 and 106. During the sampling period, i.e.,the preassigned time slot, the contents of the transceivers areinterchanged by means of the transmission gates and common highways, thelatter being shared by all of the stations terminating on the switchunit. As indicated, the contents of the transceivers are applied to thedecoders after the sampling period has terminated.

As noted in FIG. 2, the preassigned time slot is divided into fouroperating intervals of 20, 40, 20 and 40 nanoseconds, respectively, fora total of 120-nanosecond time slot. The sequence of operationsoccurring during the time slot is noted in FIG. 2. Thus, in the first 20nanoseconds, registers 116 and 121 on the common highways are cleared orreset to zero preparatory to the receipt of the next coded signalsample. Thereafter, in a 40-nanosecond interval, the signal sampleavailable in transceiver 105 is transferred to register 116 viatransmission gate 107 and common highway 115. Similarly, during this40-nanosecond interval the signal sample in transceiver 106 istransferred to register 121 via transmission gate 108 and common highway120. In the following 20-nanosecond interval transceivers 105 and 106are cleared or reset to zero preparatory to the return of the signalsamples in the final 40- nanosecond interval of the time slot. In thisinstance, the signal sample in register 116 is transferred totransceiver 106 via highway 115 and gate 108' while the sample inregister 121 is transferred to transceiver 105 via highway 120 and gate107'.

During the next time slot the same operation is repeated for a dilferentpair of subscribers, their identities, or rather the identities of thegates to be enabled for their interconnection, being stored in the nextsequential posi tion in switch store 150. Thus switch store 150 cyclesthrough all of the control words corresponding to time slots insequence. If the sampling rate is 8 kilohertz, a scan through switchstore 150 will be completed in 125 microseconds. Since each time slotrequires only 120 nanoseconds, more than 1,000 time slots can beaccommodated within the 125-microsecond frame interval. This thencontrasts with the 3.2-microsecond time slot in the aforementionedGebhardt et al. arrangement which can accommodate a mere 24 time slotsin the same frame interval.

The arrangement described in regard to FIG. 1 may be termed a four-wiresystem with all transmission unilateral. An alternative arrangementwhich proves advantageous for conference connections, as describedhereinafter, may be termed a two-wire system in which signal samples aretransmitted in both directions through the same transmis- W sion path.Such an arrangement is depicted in FIG. 3. The

apparatus and interconnecting facilities are essentially the same asthose depicted in FIG. 1 with the addition of a subtraction circuit ineach line, such as subtractors 300 and 301 associated with therespective stations 1 and 100-n. This circuit performs a subtractionoperation between the coded outgoing signal sample and the composite ofthe incoming and outgoing signal samples received through the switchingnetwork. The resultant of the subtraction is the desired incoming codedsignal sample. Also included in this arrangement is adder circuit 305which, as its name implies, serves to add the coded signal sample fromeach station engaged in a conversation. The resultant sum is applied tosum buffer 310. All of these circuits are straightforward and easilyimplemented with arrangements available in the art.

The manner of operation of the arrangement depicted in FIG. 3 isindicated in the time chart, FIG. 4. The time slot in this instance isdivided into five distinct intervals of, respectively, 20, 40, 20, 20and 40 nanoseconds for a total time slot interval of nanoseconds. Thesequence of operations may be traced through the network of FIG. 3.considering again the example of a conversation in progress betweenstations 100-1 and 100-n.

In the first ZO-nanosecond interval, highway registers 116 and 121 andsum buffer 310 are cleared by resetting them to zero. The encodedsignals stored in transceivers 105 and 106, representing signal samplesfrom the respective stations 100-1 and 100n, are thereafter gated to therespective highway registers 116 and 121 via bidirectional gates 107 and108. This action occurs during the second distinct interval in the timeslot which has a duration of 40 nanoseconds, FIG. 4. 1n the followingZO-nanosecond interval, the signal samples currently stored in registers116 and 121 are applied simultaneously to adder 305 from which theresultant sum is immediately transferred to sum buffer 310 forshort-term storage. Registers 116, 121 and transceivers 105, 106 arereset during the next ZO-nanosecond interval. Then in the final 40nanoseconds, the composite signal sample is transferred from sum buffer310 to both transceivers 105 and 106 via the same transmission pathsoccupied during the initial transfer from the transceivers to thehighway registers.

Subsequent to the assigned time slot, the composite signal is applied tosubtractors 300 and 301 such that the signal sample derived from station1001 is converted to analog form in decoder 104 and applied to station100-n. Similarly, the signal sample derived from station 100n isconverted to analog form in decoder 102 for application to station100-1.

This sequence of operations requires 20 nonaseconds more than utilizedin the arrangement according to FIG. 1. At the sampling rate of 8kilohertz, the number of time slots which can be accommodated by switchstore is reduced to 893. However, the arrangement according to FIG. 3 isparticularly adaptable to multiparty conference connections. Thus, forexample, as noted in FIG. 5, four telephone lines 500-503 areinterconnected for a conference. Such an arrangement, of course,requires that each conferee receive the signal samples provided by allof the other conferees. For this purpose additional sum buffer 510 isconnected to receive the cumulative signal sum stored in buffer 31. Ingeneral, with n conferees, n-l time slots are required if n is even, andn time slots are required if n, is odd. Thus with the four conferees500-503, three time slots are required. For example, as noted in themessage sequence in switch store 150, samples are transferred from lines500 and 501 via gates 107 and 521, respectively, in a first time slotand from lines 502 and 503 via gates 520 and 108, respectively, in asecond time slot. The samples from lines 500 and 501 are added togetherin adder 305 and their sum stored in additional sum buffer 510 viabuffer 310 during the first time slot assigned to this conference.During the second conference time slot the contents of buffer 510 isadded to the samples received from lines 502 and 503 in adder 305 andthe resultant stored in buffer 310. Also, during the second conferencetime slot, this resultant composite signal is transferred from buffer310 to conference lines 502 and 503 via the same bilateral gates 520 and108 from which the signal samples previously were received from thecorresponding conference lines. In order to apply this resultantcomposite signal to lines 500 and 501 as well, an additional conferencetime slot is employed, as illustrated in FlG. 5. In this instance thecomposite signal is retained in buffer 310 for transfer to lines 500 and501 during the third conference time slot via bidirectional gates 107and 521. All conference time slots appear in sequence during therepetitive cycle of time slots.

An alternative arrangement for multiparty conferences is illustrated inFIG. 6. In this instance an additional memory register 601 andtranslators 602 and 603 are included. The bidirectional gate controlsignals in memory 150 are applied to translators 130 and 131 during thefirst conference time slot in order to transfer signals from lines 500and 501 and simultaneously, these gate control signals are stored inregister 601. During the second conference time slot, therefore. theresultant composite signal may be gated simultaneously to all conferencelines 500 503 by applying the contents of registers 140 and 601simultaneously to appropriate ones of the bidirectional gates. Combiningcircuits 600 and 604 assure that the composite signal will betransmitted to each of the conference lines as required. Thisalternative, of course, requires additional circuitry but has theadvantage of conserving available time slots where speed is of theessence.

Underlining the conference arrangements considered in this illustrativeembodiment of our invention i the transfer of signal samples in digitalform rather than in the analog form as employed in the prior art. Thevirtue of digital transfer is the preservation of the same signal levelthroughout the transfer operation for all conferees. In the prior artarrangement, the energy in the analog signal is divided among allconferees. Thus, if one conferee is speaking to two others, the energyof his voice, at best, is divided equally between the two recipients.assuming, of course, that the line circuits of all conferees areperfectly terminated. Each additional confcrce will reduce the signallevel accordingly. In such an arrangement it is impractical to permitmore than four conferees in a single conference without insertingadditional gain. In accordance with this illustrative embodiment of ourinvention, with the signal in digital form. any number of conferees maybe included in the conference connection without signal loss ordegradation.

As noted in FIGS. 79, redundancy in the adder and buffer circuitrytogether with a partition of the switch store translation circuitry canassure continuous service at reduced capacity upon the occurrence of anysingle fault and certain multiple faults. Thus in FIG. 7, each of thetwo highways 115 and 120 is provided with its own adder and buffercircuitry, viz, adder 700, sum buffer 701 and additional sum buffer 702for highway 115 and adder 710, sum buffer 711 and additional sum buffer712 for highway 120. The switch store, in turn, is divided into twoequal sections 150 and 150' with separate access to each section, thusforming two independent stores of the same word length. The two storesare run in synchronism with translator 130 associated with section 150and translator 131 associated with section 150'. If a failure occurs inone of the highways 115 or 120 or in one of the adders 305, 700 or 710.the other highway and the associated section of the switch store isavailable to continue the operation of the system on its own. Forexample, if highway 115, adder 305 and switch store section 150 allexhibit failures simultaneously, the system may be continued in serviceutilizing highway 120, adder 710 and switch store section 150'.

Alternative arrangements for maintaining this in-service condition areshown in FIGS. 8 and 9. Thus as noted in FIG. 8, three time slots areassigned to each conversation which, of course, means that the systemcapacity is reduced to one-third of its normal capacity. In thisinstance a sample from subscriber line 500 is stored in additional sumbuffer 712 during the first time slot, and in the second time slot :1sample from line 503 is added to the sample in butter 712 and theresultant sum returned to line 503. Finally, in the third time slotassigned to this conversation, the resultant sum contained in buffer 711is returned to line 500.

The alternative method, as illustrated in FIG. 9, provides a memorybuffer 901 and additional translator 902 which permits continued serviceon a two-time slot per conversation basis. Again a sample from line 500is stored in additional sum buffer 712 during the first time slot, andat the same time the information for enabling bidirectional gate 109 istransferred to memory buffer 901. In the second time slot, therefore,the signal from line 503 is added to the contents of buffer 712 and theresultant sum applied to both lines 500 and 503 simultaneously via gates107 and 108.

What is claimed is:

1. A time division switching system comprising a plurality of lines,first and second common transmission highways and means forinterconnecting calling and called ones of said lines via said highwaysduring an assigned time slot in a repetitive cycle of time slots, saidinterconnecting means comprising means for simultaneously transferring asignal sample from each of said calling and called lines to said firstand second highways respectively, means for adding said calling linesample to said called line sample, means for transferring the resultantsignal summation to said calling and called lines, means for subtractingthe signal sample developed in each of said calling and called linesfrom said resultant signal summation and means for applying theresultant difference sig nals t0 the respective calling and calledlines.

[2. A time division communication system comprising a plurality oflines, a plurality of common highways accessible from said plurality oflines, means for transferring a signal sample from each of saidplurality of lines to a corresponding preselected one of said pluralityof highways during a first portion of a time slot in a repetitive cycleof time slots and means operative in a second portion of the same timeslot for transferring said signal samples from said plurality ofhighways to corresponding, predetermined ones of said plurality oflines] [3. A time division communication system in accordance with claim2 wherein each of said highways comprises register means, said signalsamples from said plurality of lines being interchanged via saidregister means during said same time slot.]

4. A time division communication system in accordance with claim [2] 1wherein each of said plurality of lines comprises means for [coding]digitally encoding and decoding said signal samples and furthercomprising means for registering said coded signal samples prior totransmission to said highways via said transferring means.

5. A time division communication system in accordance with claim [2] 1wherein each of said highways comprises register means, and furthercomprising means interconnecting said highway register means forcombining said signal samples and buffer storage means for applying theresultant combined signal sample from said combining means to each ofsaid plurality of lines via said register means in each of saidhighways. and wherein each of said lines comprises means for subtractingthe signal sample produced by the corresponding line from said resultantcombined signal sample] 6. A time division communication system inaccordance with claim 5 and further comprising means for storing thecontent of said buffer storage means and means for applying the contentof. said storing means to said combining means during a succeeding timeslot.

[7. A communication system comprising a plurality of stations, commonhighway means, means for coding signal samples from the associatedstations, means for storing said signal samples, switching means, andmeans for enabling said switching means during distinct successive timeintervals in a repetitive cycle to transfer said signal samples to andfrom said highway means, said coding means being connected between saidstoring means and each of said stations, said switching means beingconnected between said storing means and said highway means, and saidhighway means comprising register means for storing said coded signalsamples intermediate transfers to and from said highway means] 8. Acommunication system [in accordance with claim 7 wherein said highwaymeans comprises] comprising a plurality of stations, common highwaymeans, including a pair of common highways, means for digitally encodingsignal samples from the associated stations, means for storing saiddigitally encoded signal samples, switching means, and means forenabling said switching means during distinct successive time intervalsin a repetitive cycle to transfer said signal samples to and from saidhighway means, said encoding means being connected between said storingmeans and each of said stations, said switching means being connectedbetween said storing means and said highway means, said highway meanscomprising register means for storing said digitally encoded signalsamples intermediate transfers to and from said highway means, andfurther comprising means interconnecting said highway register means foradding together said stored samples, and means for applying theresultant sum to said highways.

9. A time division conferencing communication system comprising aplurality of lines, a plurality of highways accessible through switchingmeans from each of said lines, each highway including a register, meansfor defining a plurality of time slots in a repetitive cycle of timeslots for signal samples transmitted between said lines over saidhighways, means for transferring signal samples from a group of saidlines among which a conferencing connection is to be established topredetermined ones of said highway registers during a first portion of atime slot, and means for transferring signal samples from said ones ofsaid highway registers to said group of said lilies in a second portionof the some time slot.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

THOMAS W. BROWN, Primary Examiner US. Cl. XR. l79-l8 BC

